Cathode, ray tubes (CRTs) are increasingly being operated at higher and higher frequencies in order to obtain higher resolution video images. High resolution video images are useful in computer aided engineering (CAE) and air traffic surveillance applications, among others. The intensity of the raster scan line producing the video image is determined by the magnitude of the cathode drive signal. In high resolution applications, the circuitry driving the cathode must be able to respond quickly and accurately to transitions of this signal.
The speed at which CRT cathodes can be driven has been limited by a number of factors. Primary among these has been the poor frequency response of prior art cathode drive circuits. This poor frequency response is due in large part to the highly reactive impedance of the cathode being driven.
The impedance of a cathode ray tube cathode is essentially purely reactive. Very little of the signal energy applied to the cathode is absorbed: most of the energy is reflected back to the source. The signals reflected from the cathode back to the source return to the cathode output amplifier and are re-reflected back to the cathode. Thus, the cathode drive circuit "rings" as energy is propagated back and forth between the cathode output amplifier and the cathode, with only a small fraction of the energy being absorbed at each reflection.
The presence of these reflections in the cathode drive circuit seriously degrades the integrity of the waveforms being applied to the cathode. The signal applied to the cathode is the vector sum of the cathode amplifier output signal plus all of the reflections re-reflected back to the cathode. The superposition of these extraneous reflections onto the original cathode drive signal causes signal degradation that increases with frequency. Such signal degradation has heretofore made high frequency operation of CRT cathodes extremely difficult.
In an exemplary high resolution application, it may be necessary to produce 256 different shades of beam intensity on the screen of a CRT. The associated cathode drive circuitry must be capable of providing to the cathode a signal which can settle to one of these 256 levels in a very short period of time. The length of time available for a signal to settle to one of the 256 levels is dependent on the bandwidth of the video being applied to the cathode. The bandwidth of the video is in turn dependent on the size of the screen and the number of lines per screen. In the case of a nineteen inch, 2000 line display, a video bandwidth of 300 megahertz is required. In order to accommodate video signals having a bandwidth of 300 megahertz, the cathode drive signal must be able to switch from one of these 256 levels to the next within less than one nanosecond. Any overshoot, undershoot, or ringing caused by reflections in the cathode drive circuit must settle to the desired value within 5 nanoseconds if the integrity of the video image is to be maintained.
There are two techniques known in the art for minimizing video signal degradation caused by reflections in the drive circuit. The first technique is to locate the cathode output amplifier immediately adjacent to the neck of the CRT, next to the cathode. By minimizing the length of the conductor connecting the output amplifier to the cathode, the length of time during which reflections ring between the cathode and the output amplifier is reduced.
Positioning the cathode output amplifier immediately adjacent the neck of the cathode ray tube, however, is very undesirable from a thermal standpoint. Both the output amplifier and the neck of the CRT must dissipate substantial amounts of heat. If the CRT neck and the output amplifier are adjacent to each other, the air around each (to which heat is normally dissipated) is in part replaced by another heat producing element. This raises the operating temperature of each element with a consequent reduction in operating lifetimes and efficiencies. These thermal problems are further exacerbated in applications that use a color CRT. In such cases, there are three cathodes being driven by three separate drive circuits, with an attendant three-fold increase in heat dissipation problems.
The second prior art technique for minimizing video signal degradation caused by reflections is to insert a power dissipating resistor between the cathode output amplifier and the cathode. The resistor attenuates each signal that passes through it and thereby causes the reflections to damp to zero at an accelerated rate. Such resistance, however, also dissipates the major portion of the drive signal, decreases circuit efficiency and again exacerbates heat dissipation problems.
By combining the techniques of locating the amplifier adjacent to the neck of the CRT and coupling the amplifier to the cathode through a resistor, the operating range of cathode drive circuits can be extended to frequencies approaching 80 megahertz. As noted, however, many high resolution display applications require bandwidths far in excess of this value.
Accordingly, a need remains for an improved method and apparatus for driving the cathode of a cathode ray tube over wide bandwidths.